Technical Field
The invention relates to an operational amplifier and a driving circuit, and particularly relates to an operational amplifier capable of improving a response speed and a driving circuit thereof.
Related Art
Operational amplifier plays an important role in design of an integrated circuit, and the operational amplifier can be used to implement a plurality of different operation functions. For example, the operational amplifier can be used to enhance driving capability of an output signal for driving a load or a next stage circuit.
Generally, the operational amplifier can be a negative feedback configuration and has unity gain. Under such configuration, an output voltage of the operational amplifier is varied along with variation of a received input voltage, and the output voltage ideally has a same voltage level with that of the input voltage. However, when the input voltage is transited (which refers to a situation that the input voltage is transited from a low voltage level to a high voltage level, or is transited from the high voltage level to the low voltage level), if the voltage level thereof is changed to be too high or too low, a part of transistors in the operational amplifier are turned off. In this case, a current amount of the operational amplifier is decreased, which causes reduction of a response speed of the operational amplifier and increase of a transition period of the operational amplifier. The transition period refers to a time period required for the output voltage of the operational amplifier that changes along with transition of the input voltage reaching a stable output.
In order to mitigate the above problem, the current operational amplifier applies a constant-gm circuit, and adopts a stable bias to control a current source, so as to implement a current compensation to an input stage circuit of the operational amplifier. However, when the operational amplifier is in a stable state (i.e. the operational amplifier stably outputs the output voltage), such structure may cause extra direct current (DC) power consumption.